The subject matter disclosed herein relates to electrochromic (EC) devices. More particularly, the subject matter disclosed herein relates to maintaining the stability of a conductive layer of an electrochromic (EC) device by placing next to the transparent conductor layer an ion barrier layer comprising good electron conducting properties.
The field of electrochromics is extensive and has been developing over the last forty years. For window glazings, electrochromic coatings serve to control the amount of light and heat passing through the glazing by user-controlled applied electrical potentials across the optical stack of the coating. Not only can electrochromic coatings for window glazing reduce the amount of room heating or air conditioning, but it can also be used for privacy. By switching from a clear state having an optical transmission of about 60-80%, to a darkened state having an optical transmission of between 0.1-10%, control of both privacy and energy flow into a room through a window can be achieved. Similarly, a transparent-to-reflective EC device has recently been developed that accomplishes similar heat and/or air conditioning savings. Because the amount of glass used for various types of windows, such as skylights, aircraft windows, residential and commercial building windows, automobile windows, is on the order of one-billion square meters per year, the amount of energy savings is substantial. See, for example, Solar Energy Materials and Solar Cells, (1994) 307-321.
During the time period that the field of electrochromics has been developing, various constructs have been proposed for electrochromic devices, including solution-phase electrochromic devices, solid-state electrochromic devices, gasochromic devices, and photochromic devices. A typical electrochromic cell generally comprises the following construction: Substrate/transparent conductive layer/counter electrode/ion conductor layer/electrochromic layer/transparent conductive layer.
Electrochromic devices require at least one transparent electrical conductor that, for example, can be composed of Indium Tin Oxide (ITO), 80-90% indium oxide with a minor amount of ITO, or fluorine-doped tin oxide (SnO2:F, also abbreviated as FTO), or aluminum doped ZnO (ZnO:Al), sometimes abbreviated as AZO. Recently, in 2006, amorphous zinc indium tin oxide (ZITO) has been disclosed. The transparent conductor is located next to either the cathodic electrochromic layer or next to the anodic counter electrode of an electrochromic device. In order for an electrochromic device to operate efficiently, the transparent conductor layer must maintain its integrity in terms of transparency and conductivity.
Various types of transparent conducting thin films have been employed, such as Indium Tin Oxide (ITO), which is the most commonly used material, although thin metal layers, and fluorine and antimony doped tin oxide, aluminum and fluorine doped aluminum oxide have been used. In any event, conductivities of less than 20Ω/□ are needed in order to produce a uniform voltage across the face of the conductive layers. Conductivities of even lower than 20Ω/□ are needed for large panes of glass measuring 3-4 feet across.
Depending on the thickness and deposition process when ITO is used, ITO can have a resistance as low as 10Ω/□ and an overall transmission of 90% in the visible-wavelength range. To achieve 90% transmission in the visible-wavelength range, however, the resistance must be >100Ω/□. Typically, ITO has a resistivity of between about 1×10−3 to about 5.0×10−4 Ω-cm. See, for example, S. A Bashar, “Study of Indium Tin Oxide (ITO) for Novel Optoelectronic Devices” Ph.D. Ph.D thesis, www.betelco.com/sb/phd/index.html, 1998, which is incorporated by reference herein.
In comparison to ITO, ZITO has an average transmission greater than 85% across the visible-wavelength range. A maximum conductivity of 6×102 S cm−1 is obtained for Zn/In/Sn atomic ratio 0.4/0.4/0.2 in the film. See, for example, K. J. Saji et al., “Optical and electrical properties of co-sputtered amorphous transparent conducting zinc indium tin oxide thin films,” Thin Solid Films, Volume 516, Issue 18, Pages 6002-6007 (2008).
The conductivity of ITO has been studied extensively and it is believed that the conductivity is due to oxygen vacancies and to the +4 oxidation state of tin providing electrons to the conduction band. The ITO stoichiometry is complex and it is believed to be a mixture of Sn+2, Sn+4 and In+3 bonded to oxygen atoms. If a reducing agent is present, more Sn+2 can be formed, which has shown to reduce the level of conductivity. As ions (Li+, H+ or Na+) are shuttled back and forth, ions may migrate into the transparent conductive layer causing loss of electron conductance in the cell itself as these ions become trapped with the ITO structure, possibly at grain boundaries or at oxygen vacancies. So loss of conductivity in the transparent conductive layer can arise from loss of ions from the EC device (becoming fixed in the ITO layer) or due to formation of Sn+2 and/or metallic particles of Sn or In.
During the manufacture of the cathodic EC layer, lithium is added by co-sputtering to insert lithium into the structure. Metallic lithium is a strong reducing agent and if it comes in contact with the ITO, reduction of the ITO can occur (formation of Sn+2 or metallic Sn or In atoms within the matrix). This results in loss of conductivity and formation of darker ITO (loss of transparency).
In a typical absorptive solid-state electrochromic device, there are five components in the optical stack, a transparent conducting layer, an electrochromic layer (cathode), an ion storage layer, a counter electrode layer (anode) and finally another transparent conductor layer. Cathodic materials, called the “Electrochromic Electrodes,” for all solid-state devices include tungsten oxide, WO3 (most common), vanadium oxide (V2O5), molybdenum oxide (MO3), niobium oxide (Nb2O3) and iridium oxide (IrO2). Anodic materials, called the “Counter Electrodes,” include nickel oxide (NiO), nickel hydroxide (Ni(OH)2, and tungsten-doped NiO. The ion layer is formed from materials that exhibit a poor electron conductor, but a good ion conductor. Examples of such materials include SiO2, TiO2, Al2O3, and Ta2O5.
If a voltage of between 1-3 V is applied between the first and second transparent conducting layers, the following reactions take place: At the anode, the following reaction takes place:
Metal Oxide or Polymer or Organic Molecule (Colored)+xM++xe−Metal Oxide or Polymer or Organic Molecule (Transparent), in which M is H+, Li+ or Na+, e is an electron, and x is an integer.
At the cathode, the following reaction takes place:
Metal Oxide or Polymer or Organic Molecule (Transparent)+xM++xe−Metal Oxide or Polymer or Organic Molecule (Colored), in which M is H+, Li+ or Na+, e is an electron, and x is an integer.
In contrast to an absorptive solid-state electrochromic cell described above, reflective devices have an electrochromic layer that change from a transparent state to a reflective state. Two types of reflective electrochromic devices have been developed: (1) devices that use reflective metals as the active electrochromic layer that, when exposed to molecular hydrogen, become transparent as metal hydrides, and (2) devices that use reflective materials as the active electrochromic layer based on antimony or bismuth that, upon insertion of lithium ions, becomes transparent by an application of a current in the visible region of the electromagnetic spectrum.
For the first type of reflective electrochromic devices, a similar reaction takes place electrochemically when the ion layer is an alkali solution layer and a voltage is applied across the cell. In this instance, the alkali water is converted to hydrogen when the voltage is about −1 V below the standard hydrogen electrode (SHE) potential. By convention, this is set at 0 V for the pressure of hydrogen at 1 atmosphere and the solution is a hydrogen acid in which the mean ionic activity is 1.0 (i.e., 2H++2e−=H2 Half cell potential is zero).
The conversion of alkali water to hydrogen is shown in the equation below:4H2O+4e−=2H2+4OH−−1.6V4OH−=2H2O+O2+4e−+0.80V2H2O=2H2 (at cathode)+O2 (at anode)−0.8V.
Placing +1.0 V across the cell will force the reaction to proceed and form hydrogen gas as a positive emf drives the reaction. Reducing the voltage to −0.6 V on the cathode (−0.2 V relative to standard hydrogen electrode) allows the reaction to reverse itself and the hydrogen is removed from the metal hydride to from a reflective metallic layer.
For the first type of reflective electrochromic devices, U.S. Pat. No. 6,647,166 B2 to Richardson discloses details of the construction of metal-to-metal hydride EC devices, the disclosure of which is incorporated by reference herein. Similarly, U.S. Pat. No. 7,042,615 B2 to Richardson, the disclosure of which is also incorporated by reference herein, discloses the concept of electrochromic devices comprising a reflective to transparent EC layer of Sb, Bi, and like materials, such as Mg, Mg—Ti, Mg—Ni, Sb—Cu, Sb—Al, as well as metal chalcogenides, such as TiS2, NbSe and tellurides, such NbTe2.
U.S. Pat. No. 5,133,594 to Hass et al. discloses use of ion blocking, optically transparent, electronically conductive layers situated between the transparent conductor layer, i.e., ITO layer, and/or between the counter electrode and the other transparent conductive layer, i.e., the other ITO layer. Hass et al. discloses that layers of ZnO, CdO or SiC are used for blocking block ion transport, i.e., lithium ions from entering the ITO matrix.
U.S. Pat. No. 5,532,869 to Goldner et al., however, disclose that although ZnO, CdO and SiC are generally satisfactory at blocking the migration of lithium ions, these materials are not sufficiently electrically conductive, thereby resulting in electrochromic windows with very poor transmissity switching behavior, very slow switching and/r relatively high voltages needed for switching. Accordingly, Goldner et al. uses two other materials, n- and p-type lithiated silicon carbide, although n-type silicon carbide was the material preferred by Goldner et al. Forming n-type silicon carbide requires the simultaneous deposition of Li2CO3 and silicon carbide (SiC). Goldner et al. accomplished this by using pellets of Li2CO3 distributed over a target of SiC or by having separate targets of SiC and Li2CO3 oriented toward the substrate each with their independently controlled RF power supply.